This invention relates generally to a radiation exposure limiting scheme and more particularly to a radiation exposure limiting scheme for reducing the radiation exposure of a physician during the operation of an imaging system.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped, or a cone-shaped, beam which is collimated to lie within an X-Y-Z volume of a Cartesian coordinate system, wherein the X-Y-Z volume is generally referred to as an “imaging volume” and usually includes a set of X-Y planes generally referred to as the “imaging planes”. An array of radiation detectors, wherein each radiation detector includes a detector element, are disposed within the CT system so as to received this beam. An object, such as a patient, is disposed within the imaging plane so as to be subjected to the x-ray beam wherein the x-ray beam passes through the object. As the x-ray beam passes through the object being imaged, the x-ray beam becomes attenuated before impinging upon the array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is responsive to the attenuation of the x-ray beam by the object, wherein each detector element produces a separate electrical signal responsive to the beam attenuation at the detector element location. These electrical signals are referred to as x-ray attenuation measurements.
In addition, the x-ray source and the detector array may be rotated, with a gantry within the imaging volume, around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and the detector array. In an axial scan, the projection data is processed so as to construct an image that corresponds to two-dimensional slices taken through the object.
One method for reconstructing an image from a set of projection data is referred to as the “filtered back-projection technique”. This process converts the attenuation measurements from a scan into discrete integers, ranging from −1024 to +3072, called “CT numbers” or “Hounsfield Units” (HU). These HU's are used to control the brightness of a corresponding pixel on a cathode ray tube or a computer screen display in a manner responsive to the attenuation measurements. For example, an attenuation measurement for air may convert into an integer value of −1000HU's (corresponding to a dark pixel) and an attenuation measurement for very dense bone matter may convert into an integer value of +3000 (corresponding to a bright pixel), whereas an attenuation measurement for water may convert into an integer value of 0HU's (corresponding to a gray pixel). This integer conversion, or “scoring” allows a physician or a technician to determine the density of matter based on the intensity of the computer display.
Once a suspicious mass, such as a tumor, cyst and/or lesion, is discovered an interventional procedure, such as a needle biopsy or a needle aspiration, is usually performed to obtain tissue samples needed to determine whether the mass is cancerous or benign. To do this, a needle controlled by a physician is guided to the mass using simultaneous images, such as fluoro images, produced by the imaging system. This allows a physician to manipulate the needle tip towards the suspected tumor tissue so as to obtain a tissue sample that may be used for analysis.
However, although an interventional procedure using an imaging system is an excellent diagnostic and evaluation tool, each time an interventional procedure is performed by a physician, the physician's hand is exposed to radiation emitted from the imaging system. As such, if a physician performs a large number of interventional procedures over time, the cumulative radiation dose exposure to the physician's hand over time may become quite large. Given that health problems are known to be related to increasing exposure to radiation there is concern within the medical community that physicians performing these procedures may be over exposed to imaging system radiation.
One method to address the problem of physician radiation dose exposure includes minimizing the emitter current of the imaging system and using special forceps to keep the physician's hands out of the radiation beam. Unfortunately, forceps have not been well received by the medical community because they restrict the tactile sensitivity and thus limits the delicate physician control required for interventional procedures. Moreover, it has been found that minimizing the emitter current of the imaging system during an interventional procedure while still providing sufficient radiation for qualitative image generation still results in a significant cumulative radiation dose to the physician repeatedly performing the interventional procedures. As such, these methods are not well suited for repeated interventional procedures.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.